It is known to level metal strip, in particular thin metal strip having a thickness up to 1 mm, by applying to it a tension of at least 70% of the yield strength being generated in the metal strip between a braking-roller set and a tension-roller set such that the strip is leveled between the braking-roller set and the tension-roller set in a leveling-roller group having multiple leveling rollers. Here, metal strip particularly means a thin metal strip having a thickness of 0.02 mm to 1.0 mm, preferably 0.05 mm to 0.5 mm.
The goal of leveling metal strip is to make the most planar possible strip. One fundamentally differentiates in practice between various types of strip irregularities. In addition to strip waviness and strip cambering, which are to be attributed to length differences over the strip width, band curves frequently occur that are differentiated into longitudinal curve (coil set) and transverse curve (crossbow). The leveling of strips is frequently performed using strip tension, e.g. in tension straightening or in stretch-bend leveling.
Thus tension-stretching apparatuses are known in which a high strip tension is generated between a braking-roller set and a tension-roller set so that eventually the stretching tension necessary for the desired straightening is achieved. In the course of the straightening procedure, the plastic lengthening of the affected strip results in reduction of the strip thickness and strip width. Thus, for example, a method is known for the continuous tension straightening of thin strips, in particular metal strips made of steel, aluminum, or the like having a strip thickness between 0.05 and 0.5 mm, according to which the strip is subjected to the stretching tension necessary for its straightening in the plastic range in a tension straightening roller pair which is interposed between the braking-roller set and the tension-roller set. Using the tension-straightening roller pair, approximately 5% to 25% of the stretching tension for the plastic straightening is generated and, using the braking-roller set and the tension-roller set, 75% to 95% of the stretching tension for the elastic or partially elastic straightening of the strip is generated. The diameter of the tension-straightening rollers is 1500 times greater than the maximum strip thickness (cf. U.S. Pat. No. 5,182,931).
High planarity may be generated using tension straightening in practice and in particular waviness and strip camber may be removed. However, because the strip typically runs off the last tension roller in the plastic range during tension straightening, significant longitudinal curve frequently remains in the strip upon tension straightening that corresponds to the strain roller diameter minus the elastic rebound. The possibility does exist of removing this longitudinal residual curve with lower tension by an adjustable correction roller, for example. However, for thin strips the necessary diameter of the correction roller would be very small in order to still allow partially plastic counter bending. Therefore, it is frequently necessary to support such a stretch-bend leveling roller against sagging in a cassette having support rollers. In high-speed apparatuses, such rollers tend to vibrate and may cause undesired chatter marks on the strip surface. The vibrations may be sufficiently damped by the use of a spray liquid, but then removal of the liquid sprayed in the course of strip cleaning entails increased apparatus and operating costs. Furthermore, the position of the correction roller must be set again for each strip thickness/strip material combination.
Alternatively, strips are frequently leveled in practice in the course of stretch-bend leveling. The strip is bent around a number of leveling rollers having small diameters and lengthened plastically by the degree of stretching by superimposing bending and strip tension, so that waviness is nearly removed. While essentially the degree of stretching is generated on the first leveling rollers, the last leveling rollers are primarily used for curve correction. However, the roller diameter is not assumed on the last leveling rollers, because optimum radii of curvature must be set via the wrap-around angle in each case for different strips. For this reason, at least the last leveling rollers are set differently for different strips. A higher outlay for putting into operation thus frequently results in practice. Moreover, the use of comparatively small roller diameters is again disadvantageous. In addition, because of the bending and the small roller diameters, relatively high residual tensions remain in the strip across its strip thickness, which may be undesirable in the further processing of the strips. For thin strips, a plurality of leveling rollers is also necessary, so that longitudinal residual curves may be removed to the desired extent.
An apparatus for leveling metal strips is known from US 2004/0020258 in which a stretch-bending stand, a correction roller setup, and a multiple-roller leveler are provided between a braking-roller set and a tension-roller set. The multiple-roller leveler has a plurality of working rollers that are supported on support rollers. All working rollers of the stretch-bending stand, the correction roller configuration, and the multiple-roller leveler are rotated by frictional engagement of the strip with the rollers. As a result, they are not driven. The diameters of the working rollers may become greater from roller to roller in the multiple roller unit. However, the diameters are comparatively small, as is typical in multiple roller leveling and/or stretch-bend leveling. In this known apparatus, setting the position of the leveling rollers and thus the depth of engagement as a function of the strip properties is provided.
The known methods (e.g. stretch-bend leveling on the one hand and tension straightening on the other hand) are also combined. Thus, a method of continuous leveling of thin metal strips, which provides tension straightening on the one hand and stretch-bend leveling on the other hand, is known (cf. U.S. Pat. No. 5,829,287).
A method of leveling metal strip in the course of stretch-bend leveling or tension straightening, to which a leveling procedure at low strip tension in a roller leveler is connected downstream, is known from U.S. Pat. No. 6,240,762.
Finally, U.S. Pat. No. 6,925,845 describes a method of and apparatus for the stretch leveling of a metal strip, the metal strip running through a braking-roller set and a tension-roller set and being subjected to a stretching tension between the two roller sets in the course of its straightening and being subjected to bending under tension in a further roller set that is provided between the braking-roller set and the tension-roller set, to increase the stretch rate. The majority of the stretching tension is generated using this further roller set. The tension rollers of the interposed roller set may have a different diameter than the rollers of the braking-roller set and the tension-roller set. The inner tension rollers of this central roller set may have a smaller diameter than the rollers of the braking-roller set and the tension-roller set.